Cold head, superconducting magnet, examination apparatus, and cryopump

ABSTRACT

A reduction in a permeability of refrigerant gas is suppressed while increasing a filling factor of regenerator material particles with respect to a stage of a cold head. A cold head includes a stage including regenerator material particle groups, and a metal mesh material partitioning the regenerator material particle groups. The metal mesh material has quadrangular mesh holes each having a length of a long side of 1/10 or more and ½ or less of each of average particle sizes of the regenerator material particle groups.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/688,475, filed on Apr. 16, 2015, which is a continuation of priorInternational Application No. PCT/JP2013/006243 filed on Oct. 22, 2013,which is based upon and claims the benefit of priority from JapanesePatent Application No. 2012-232936 filed on Oct. 22, 2012; the entirecontents of all of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a cold head, asuperconducting magnet, an examination apparatus, and a cryopump.

BACKGROUND

In recent years, a superconducting technology has been remarkablydeveloped, and various types of examination apparatus such as a MagneticResonance Imaging (MRI) apparatus and a Nuclear Magnetic Resonance (NMR)apparatus have been used. In order to use the superconductingtechnology, a cryogenic temperature of 10 K or less, and further, 4 K orless is required to be realized. In order to realize such a cryogenictemperature, a refrigerator called as a cold head is used.

There are various types of cold head such as one of Gifford-McMahon type(GM type), one of Stirling type, and one of pulse type. In any one ofthe types of cold head, to achieve the cryogenic temperature, aregenerator material is filled in a regenerator container called as astage.

Depending on a design of the cold head, the stage is sometimes formed ofone stage, and is sometimes divided into a plurality of stages such astwo stages. Helium gas is passed through the stage, and a specific heatper volume of the regenerator material is utilized to obtain thecryogenic temperature.

One example of a cold head for obtaining the cryogenic temperature of 4Kor less has a three-layer structure including a lead regeneratormaterial, a HoCu₂ regenerator material, and a GOS regenerator material(gadolinium oxysulfide regenerator material) filled in a second stage(second-stage regenerator container).

The above-described cold head can obtain the cryogenic temperature bycausing an adiabatic expansion of refrigerant gas such as helium gas. Asdescribed above, in the cold head for obtaining the cryogenictemperature, a plurality of types of regenerator materials are filled inlayers in the stage.

When the plurality of types of regenerator materials are used, since aspecific heat peak temperature of each of the materials is different, itis not possible to mix and use the regenerator materials. Accordingly,when the plurality of types of regenerator materials are used, thematerial layers are partitioned by a metal mesh material.

As the regenerator material, for example, a regenerator materialparticle group with aligned particle size in which a proportion ofparticle having a particle size of not less than 0.01 mm nor more than 3mm is 90% or more, and a proportion of particle having an aspect ratioof 5 or less is 90% or more, is used.

When the metal mesh material is used to partition the plurality of typesof regenerator material particle groups, a problem in which mesh holesof the metal mesh material are clogged with the regenerator materialparticles, has arisen. If the mesh holes are clogged with theregenerator material particles, a permeability of refrigerant gas islowered.

In the regenerator operation using the regenerator material particles,the adiabatic expansion due to a specific heat per volume of theregenerator material particles is utilized, so that it is preferable tofill as many regenerator material particles as possible in the stage.For this reason, the regenerator material particle groups are filled soas to be closely contacted to the metal mesh material. In addition tothat, the permeability of the refrigerant gas has to be secured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an example of structure of a secondstage of a cold head of an embodiment.

FIG. 2 is a diagram illustrating an example of structure of the secondstage of the cold head of the embodiment.

FIG. 3 is a diagram illustrating an example of structure of a metal meshmaterial.

FIG. 4 is a diagram illustrating one example of a cross section of themetal mesh material.

FIG. 5 is a diagram illustrating another example of the cross section ofthe metal mesh material.

FIG. 6 is a diagram illustrating an example of structure of a stack ofmetal mesh materials.

DETAILED DESCRIPTION

A cold head of an embodiment includes a stage including: regeneratormaterial particle groups; and a metal mesh material partitioning theregenerator material particle groups. The metal mesh material hasquadrangular mesh holes. A length of a long side of the mesh hole is1/10 or less and ½ or more of each of average particle sizes of theregenerator material particle groups.

A cold head of the present embodiment has a first stage and a secondstage. As a type of the cold head, various types such as a GM type, aStirling type, or a pulse type can be employed. In any one of the typesof cold head, to achieve the cryogenic temperature of 10 K or less, andfurther, 5 K or less, regenerator materials are filled in the firststage and the second stage. As the regenerator material for the firststage, a copper mesh material, for example, can be used.

An example of structure of the second stage will be described whilereferring to FIG. 1 and FIG. 2 . FIG. 1 and FIG. 2 are diagrams eachillustrating an example of structure of the second stage of the coldhead.

The second stage illustrated in FIG. 1 is a stage of two-layer type, andhas a regenerator material particle group 1, a regenerator materialparticle group 2, a regenerator container 3, and metal mesh materials 4.The second stage illustrated in FIG. 2 is a stage of three-layer type,and has a regenerator material particle group 1, a regenerator materialparticle group 2, a regenerator container 3, metal mesh materials 4, anda regenerator material particle group 5. As illustrated in FIG. 1 andFIG. 2 , the second stage has two or more, and further, three or more ofregenerator material-filled layers partitioned by the metal meshmaterials 4 and having the plurality of regenerator material particlegroups respectively filled therein.

In the second stage illustrated in FIG. 1 , the regenerator materialparticle group 1 and the regenerator material particle group 2 arefilled with the metal mesh materials 4 provided therebetween. The secondstage is partitioned into two layers of a first regeneratormaterial-filled layer having the regenerator material particle group 1filled therein and a second regenerator material-filled layer having theregenerator material particle group 2 filled therein.

In the second stage illustrated in FIG. 2 , the regenerator materialparticle group 1, the regenerator material particle group 2, and theregenerator material particle group 5 are filled with the metal meshmaterials 4 provided among the groups. The second stage is partitionedinto three layers of a first regenerator material-filled layer havingthe regenerator material particle group 1 filled therein, a secondregenerator material-filled layer having the regenerator materialparticle group 2 filled therein, and a third regenerator material-filledlayer having the regenerator material particle group 5 filled therein.As described above, the second stage may have a structure in which twoor more, or three or more of regions partitioned by the metal meshmaterials 4 are provided. Note that in each of FIG. 1 and FIG. 2 , thestructure of partitioning the second stage into two layers or threelayers is exemplified, but, the present invention is not limited tothis, and it is also possible to partition the second stage into fourlayers or more.

The metal mesh material 4 partitions the regenerator material particlegroup 1, the regenerator material particle group 2, and the regeneratormaterial particle group 5. The metal mesh material 4 may also be broughtinto contact with at least one of the regenerator material particlegroup 1, the regenerator material particle group 2, and the regeneratormaterial particle group 5, for example.

An example of structure of the metal mesh material 4 will be describedwhile referring to FIG. 3 . FIG. 3 shows an example of structure of themetal mesh material 4. The metal mesh material 4 shown in FIG. 3 hasmesh holes 6 and metal wires 7.

A shape of the mesh hole 6 is preferably a quadrangular shape. Here, thequadrangular shape indicates, for example, a square shape or arectangular shape. By setting the shape of the mesh hole 6 to thequadrangular shape, when the regenerator material particle groups arefilled, it is possible to form a gap in each of the mesh holes 6. Thismakes it possible to suppress the mesh holes 6 from being clogged withthe regenerator material particle groups, so that a reduction in apermeability of refrigerant gas can be suppressed. Note that the metalmesh material 4 normally has a circular shape, as illustrated in FIG. 3. Accordingly, the mesh hole 6 at an end portion does not have to have aquadrangular shape.

When the shape of the mesh hole 6 is the quadrangular shape, a length ofa long side of the mesh hole 6 is preferably ½ or less of each ofaverage particle sizes of the regenerator material particle groupspartitioned by the metal mesh material 4. For example, out of ahorizontal width T1 of the mesh hole 6 and a vertical width T2 of themesh hole 6 illustrated in FIG. 3 , the longer width corresponds to theaforementioned long side.

If the length of the long side of the mesh hole 6 exceeds ½ of theaverage particle size of the regenerator material particle grouppartitioned by the metal mesh material 4, the regenerator materialparticle easily passes through the mesh hole 6, so that there is apossibility that different types of regenerator material particle groupsare mixed, and the refrigerating capacity deteriorates. In order to forman appropriate gap in each of the mesh holes 6 when the regeneratormaterial particle groups are filled, the length of the long side of thequadrangular mesh hole 6 is preferably ½ or less, more preferably ⅓ orless of each of the average particle sizes of the regenerator materialparticle groups partitioned by the metal mesh material 4. When thelength of the long side is too short, the mesh hole 6 is sometimescompletely blocked by the regenerator material particle. The length ofthe long side is preferably 1/10 or more of the average particle size ofthe regenerator material particle groups partitioned by the metal meshmaterial 4.

The metal wires 7 have a structure of being woven in a mesh form, forexample. At this time, a region surrounded by the metal wires 7corresponds to the mesh hole 6. Note that the structure of the metalmesh material 4 is not necessarily limited to the woven structure.

A wire diameter of the metal wire 7 is preferably not less than 20 μmnor more than 90 μm. The wire diameter of less than 20 μm becomes a maincause of increasing cost, since a strength of the metal mesh material 4is lowered, and it becomes difficult to manufacture the metal meshmaterial 4. If the wire diameter exceeds 90 μm, the permeability of therefrigerant gas is lowered. The wire diameter of the metal wire 7 ispreferably not less than 20 μm nor more than 90 μm, and more preferablynot less than 40 μm nor more than 60 μm.

FIG. 4 illustrates one example of a cross section of the metal meshmaterial 4. As illustrated in FIG. 4 , by making the metal mesh material4 have a structure in which metal wires 7B are woven with respect to atop and a bottom of the metal wire 7 in an alternate manner, the shapeof the mesh hole 6 can be set to a three-dimensional shape with concaveand convex. By setting the shape of the mesh hole 6 to thethree-dimensional shape, it is possible to make a contact surfacebetween the regenerator material particle group and the metal meshmaterial 4 to be a three-dimensional surface. The clogging of the meshholes 6 caused by the regenerator material particles is effectivelysuppressed, resulting in that the reduction in the permeability of therefrigerant gas can be suppressed.

FIG. 5 illustrates another example of the cross section of the metalmesh material 4. As illustrated in FIG. 5 , it is also possible to setthe shape of each of the metal wire 7A and the metal wire 7B to a flatplate shape. When the wire of the flat plate shape is used, a thickerwidth is set to a wire diameter. Even if the wire of the flat plateshape is employed, the wire diameter of the metal wire 7 is preferablynot less than 20 μm nor more than 90 μm.

The metal mesh material 4 is preferably a copper mesh material, forexample. The copper mesh material is used as the regenerator materialfor the first stage as described above, and even if it is provided inthe second stage, it is possible to suppress an influence on therefrigerating capacity. Further, since the material has a high springcharacteristic, it is difficult to be broken even if a stress is appliedthereto for increasing a filling density of the regenerator materialparticles. Further, it is possible to mitigate a vibration during anoperation of the cold head, and a vibration when the refrigerant gas isflowed. As a result of this, it is possible to suppress the breakage ofthe regenerator material when the cold head is operated for a longperiod of time.

Further, the second stage is preferably provided with a plurality of themetal mesh materials 4. At this time, the plurality of the metal meshmaterials 4 are stacked. By using the stack of two or more of the metalmesh materials 4, it is possible to further increase the springcharacteristic. Accordingly, a stress when filling the regeneratormaterial particle groups is increased, resulting in that a process ofincreasing the filling density can be shortened. Note that when thenumber of the metal mesh materials 4 is too large, a space in the stagein which the regenerator material particle groups are filled becomesnarrow, so that the number of the metal mesh materials 4 is preferably 5or less.

By using the stack of the two or more of the metal mesh materials 4, itis possible to further suppress the breakage of the regenerator materialwhen the cold head is operated for a long period of time. As a result ofthis, the refrigerating capacity can be maintained even if the cold headis continuously operated for 25000 hours or more, and further, 35000hours or more.

The plurality of the metal mesh materials 4 are preferably stacked sothat positions of the mesh holes do not match (the positions aredisplaced). FIG. 6 is a diagram illustrating an example of structure ofa plurality of metal mesh materials. A metal mesh material 4-2illustrated in FIG. 6 is stacked on a metal mesh material 4-1. At thistime, a mesh hole of the metal mesh material 4-2 is overlapped withmetal wires of the metal mesh material 4-1. As described above, bymaking the metal mesh material have a shape of being woven in thethree-dimensional manner, it is possible to suppress the reduction inthe permeability of the refrigerant gas while suppressing the cloggingof the mesh holes of the metal mesh materials caused by the regeneratormaterial particles.

In order to displace the positions of the mesh holes in the stack of theplurality of the metal mesh materials 4, it is effective to use aplurality of metal mesh materials with different shapes of mesh holes.For example, in the stack of the plurality of the metal mesh materials4, a length of one side of the mesh hole is preferably different betweenor among the metal mesh materials 4. Accordingly, when the cold head iscontinuously operated, even if the regenerator material is broken due toa vibration during the operation and a gas pressure, the clogging of themetal mesh materials is suppressed. As a result of this, therefrigerating capacity can be maintained for a long period of time. Theabove is the explanation regarding the example of structure of the metalmesh material 4.

The regenerator material particle groups (the regenerator materialparticle group 1, the regenerator material particle group 2, and theregenerator material particle group 5) illustrated in FIG. 1 and FIG. 2may also be regenerator material particle groups of mutually differenttypes.

Each of the regenerator material particle groups filled in therespective regenerator material-filled layers partitioned by the metalmesh materials in the second stage, is preferably at least one selectedfrom a lead regenerator material particle group, a holmium-copperregenerator material particle group, an erbium-nickel regeneratormaterial particle group, an erbium-cobalt regenerator material particlegroup, a gadolinium oxysulfide regenerator material particle group, anda gadolinium-aluminum oxide regenerator material particle group. When aplurality of types of regenerator material particle groups are used, itis preferable to select a combination of the groups with the samespecific heat peak temperature or a combination of the groups whosespecific heat peak temperatures are decreased in sequence.

For example, one of the plurality of types of regenerator materialparticle groups can be set to the holmium-copper regenerator materialparticle group or the erbium-nickel regenerator material particle group.As the holmium-copper regenerator material particle group or theerbium-nickel regenerator material particle group, one excellent in aform factor R and a strength, has been developed, as disclosed inPublication of Japanese Patent No. 3769024.

The holmium-copper regenerator material particle is preferably made of,for example, HoCu₂ or HoCu. The erbium-nickel regenerator materialparticle is preferably made of, for example, ErNi or Er₃Ni.

Each of average particle sizes of the regenerator material particlegroups is preferably not less than 200 μm nor more than 300 μm. Bysetting the average particle size to not less than 200 μm nor more than300 μm, it becomes easy to prepare regenerator material particles withhigh mechanical strength. Further, also when the regenerator materialparticle groups are filled in the second stage, it is possible to formgaps for maintaining the permeability of the refrigerant gas whileincreasing the filling density.

In the regenerator material particle groups, a number proportion of theregenerator material particle having a particle size which falls withina range of not less than 150 μm nor more than 350 μm, is preferably 95%or more. If it is a large number of regenerator material particles eachhaving a particle size of less than 150 μm, this causes the clogging ofthe mesh holes 6 of the metal mesh material 4. Further, if it is a largenumber of regenerator material particles each having a particle size ofgreater than 350 μm, the filling density of the regenerator material islowered, resulting in that the refrigerating capacity deteriorates. Forthis reason, the number proportion of the regenerator material particlehaving the particle size which falls within the range of not less than150 μm nor more than 350 μm is, for example, preferably 95% or more,more preferably 98% or more, and still more preferably 100%.

A shape of the regenerator material particle configuring each of theregenerator material particle groups is preferably a spherical shape ora shape approximated to the spherical shape, in order to increase thefilling density.

When a circumferential length of a projected image of the regeneratormaterial particle configuring each of the regenerator material particlegroups is set to L, and an actual area of the projected image is set toA, a proportion of the regenerator material particle in which a formfactor R represented by L²/4πA exceeds 1.5 is preferably 5% or less. Ifthe form factor R represented by L²/4πA is 1.5 or less, this means thata sphericity of the regenerator material particle is high, and a surfaceis smooth. Even if the particle is seemingly sphere, when a large numberof very small concave and convex exist on the surface, the form factor Rsometimes exceeds 1.5.

In order to perform the filling so as not to generate unnecessary gapsbetween the metal mesh materials 4 and the regenerator material particlegroups, for example, it is preferable that a small vibration is appliedto the regenerator container to fill the regenerator material particlesso that the gap between the regenerator material particles becomessmall, and thereafter, the metal mesh materials 4 are pressed againstthe particles while applying a stress, and the materials are fixed. Asdescribed above, in order to efficiently fill the regenerator materialparticle groups, it is effective to apply the vibration and the stress.

When the vibration and the stress are applied to the regeneratormaterial particles, it is required that the regenerator materialparticle has a high mechanical strength. As one of methods of improvingthe mechanical strength of the regenerator material particle, there canbe cited a method of making the form factor R to be 1.5 or less. Byeliminating the very small concave and convex on the surface, it ispossible to improve the mechanical strength. A proportion of theregenerator material particle whose form factor R exceeds 1.5 ispreferably 5% or less, more preferably 2% or less, and still morepreferably 0%. Further, the form factor R of each of the regeneratormaterial particles is preferably 1.2 or less.

By reducing the form factor R of each of the regenerator materialparticles, the mechanical strength of the regenerator material particlesis increased, and it is possible to fill the regenerator materialparticles so as not to generate the unnecessary gaps between theparticles and the metal mesh materials 4, resulting in that therefrigerating capacity can be improved.

When the plurality of types of regenerator material particle groups areused, in at least one regenerator material particle group, a proportionof particle having the form factor R of 1.5 or less is preferably 5% orless. In particular, it is preferable that the regenerator materialparticle group to be arranged on a lower side in the second stagesatisfies the above-described proportion. This is because when theregenerator material particle groups are filled in the second stage, theregenerator material particle group to be arranged on the lower side isfirstly filled, and thus the regenerator material particle group to bearranged on the lower side receives greater vibration and stress, whencompared to a vibration and a stress with respect to the regeneratormaterial particle group to be arranged on an upper side. Note that whenthe plurality of types of regenerator material particle groups are used,it is more preferable that a proportion of particle having the formfactor R of 1.5 or less is 5% or less in all of the regenerator materialparticle groups.

In the regenerator material particle groups, it is possible toseparately conduct the control of the particle size and the control ofthe form factor R described above, it is preferable to conduct both ofthe controls in a combined manner.

In the cold head of the present embodiment, the shape of the mesh holeof the metal mesh material is adjusted. This makes it possible tosuppress the clogging of the mesh holes caused by the regeneratormaterial particle groups, so that the reduction in the permeability ofthe refrigerant gas is suppressed. For this reason, it is possible tofill the plurality of types of regenerator material particle groups inthe stage of the cold head to improve the refrigerating capacity.Therefore, a reliability of a superconducting magnet, various types ofapparatus such as an examination apparatus and a cryopump each includingthe cold head of the present embodiment, is improved.

As the examination apparatus, an MRI apparatus, an NMR apparatus and thelike can be cited. For example, the MRI apparatus is a medical equipmentwhich can take a photograph of human body in vertical and horizontaldirections by utilizing a magnetism. Presently, the MRI apparatus canobtain a clear image at a level of equal to or greater than that of anX-ray Computed Tomography (CT) apparatus, and is used for angiography,and for photographing to check the presence/absence of aneurism in thebrain or brain tumor.

The photographing in the MRI apparatus is conducted not only in aroutine checkup but also in an emergency medical examination, as amatter of course. For this reason, there is a need to make the MRIapparatus to be constantly operated so that it is possible to performphotographing at any time. In order to make the MRI apparatus to beconstantly operated, it is required to make a superconducting magnet forobtaining the cryogenic temperature and a cold head including thesuperconducting magnet to be in an operation state. In the cold head ofthe present embodiment, the filling density of the regenerator materialparticles is improved, and in addition to that, the reduction in thepermeability of the refrigerant gas is suppressed. Accordingly, therefrigerating capacity possessed by the regenerator material particlescan be maintained over a long period. Therefore, it is possible toobtain a long-term reliability of not only the cold head but also thesuperconducting magnet, and the various types of apparatus such as theexamination apparatus and the cryopump each including the cold head.

EXAMPLES Example 1 to Example 10, Comparative example 1

Samples including regenerator material particles presented in Table 1were prepared as regenerator material particle groups. Note that inTable 1, a Pb particle indicates a lead regenerator material particle. AHoCu₂ particle indicates a holmium-copper regenerator material particle.An Er₃Ni particle indicates an erbium-nickel regenerator materialparticle. A GOS particle indicates a gadolinium oxysulfide regeneratormaterial particle. A GAP particle indicates a gadolinium-aluminum oxideregenerator material particle. A HoCu particle indicates aholmium-copper regenerator material particle. An Er₃Co particleindicates an erbium-cobalt regenerator material particle. Further, thecontrol of the form factor R and the control of the particle size areconducted by a classification based on form.

TABLE 1 Form Factor R Number Number Number Number Proportion ofProportion Proportion Proportion Particle Having Sample of RegeneratorSatisfying Satisfying Satisfying Average Particle Size of RegeneratorMaterial R > 1.5 1.5 ≥ R > 1.2 1.2 ≥ R Particle 150 μm to 350 MaterialParticle Group (%) (%) (%) Size (μm) μm (%) Sample 1 Pb Particle 0 4 96250 100 Sample 2 HoCu₂ Particle 1 2 97 200 98 Sample 3 HoCu₂ Particle 00 100 260 100 Sample 4 HoCu₂ Particle 10 20 70 380 70 Sample 5 Er₃NiParticle 2 1 97 270 98 Sample 6 GOS Particle 0 0 100 240 100 Sample 7GAP Particle 2 3 95 250 98 Sample 8 HoCu Particle 1 5 94 240 100 Sample9 Er₃Co Particle 3 7 90 270 100

Next, samples of copper mesh materials presented in Table 2 wereprepared as metal mesh materials. Note that “presence” of wovenstructure indicates a sample of metal mesh material formed by weavingcopper wires, and “absence” of woven structure indicates a sample ofmetal mesh material formed by performing etching on a metal plate madeof copper.

TABLE 2 Wire Copper Diameter Mesh Mesh Hole of Copper Woven MaterialShape T1 (μm) T2 (μm) Wire (μm) Structure Sample A Rectangular 120 100100 Absence Shape Sample B Square Shape 100 100 85 Presence Sample CSquare Shape 70 70 50 Presence Sample D Square Shape 40 40 30 PresenceSample E Circular Shape Diameter: 100 70 Absence

Next, the samples of the regenerator material particle groups (thesample 1 to the sample 9) and the samples of the metal mesh materials(the sample A to the sample E) were combined as presented in Table 3, tothereby create a two-layer structure or a three-layer structure in asecond stage of each of cold heads. At this time, the regeneratormaterial particle groups were filled while applying a vibration so thatthe filling factor became as high as possible. Between the respectiveregenerator material particle groups, three of the samples of the metalmesh materials were stacked to be arranged.

The permeability of the refrigerant gas was examined regarding therespective examples and the comparative example. In the examination ofthe permeability of the refrigerant gas, when a quantity of flowed gasin the comparative example 1 was set to 100, a cold head in which aquantity of flowed gas was 120 or more and less than 141 were indicatedby “moderate”, and a cold head in which a quantity of flowed gas was 141or more were indicated by “good”. Results thereof are presented in Table3.

TABLE 3 First Second Third Regenerator Regenerator Regenerator MaterialMaterial Material Metal Mesh Filling Particle Group Particle GroupParticle Group Material Factor (%) Permeability Example 1 Sample 1Sample 2 — Sample A 62 Moderate Example 2 Sample 1 Sample 3 — Sample B62 Good Example 3 Sample 1 Sample 3 Sample 6 Sample C 63 Good Example 4Sample 1 Sample 3 Sample 7 Sample D 62 Good Example 5 Sample 2 Sample 4— Sample B 55 Moderate Example 6 Sample 3 Sample 5 — Sample C 63 GoodExample 7 Sample 3 Sample 6 — Sample D 63 Good Example 8 Sample 1 Sample3 Sample 3 Sample B 62 Good Example 9 Sample 8 Sample 3 Sample 6 SampleC 62 Good Example 10 Sample 8 Sample 9 Sample 6 Sample C 62 Good Comp.Exam 1 Sample 1 Sample 2 — Sample E 62 —

It was confirmed that the permeability of the refrigerant gas was highin the cold head related to each of the example 1 to the example 10. Thepermeability of the refrigerant gas was high in each of the example 2 tothe example 4, and each of the example 6 to the example 8, inparticular. As described above, the reduction in the permeability of therefrigerant gas is suppressed by optimizing the shape of the regeneratormaterial particle and the shape of the mesh hole of the metal meshmaterial. For this reason, it is possible to improve the refrigeratingcapacity of the cold head by making the best use of the performance ofthe regenerator material.

Example 11 to Example 13

The regenerator material particles in a combination same as that of theexample 3 (the sample 1, the sample 3, and the sample 6) were prepared.Next, three types of the sample B to the sample D were prepared as themetal mesh materials, and by stacking the materials as presented inTable 4, cold heads related to the example 11 to the example 13 wereformed.

Regarding the cold heads related to the example 3, and the example 11 tothe example 13, a maintenance ratio of the refrigerating capacity afterthe cold heads were continuously operated was examined. Regarding themaintenance ratio of the refrigerating capacity, each of therefrigerating capacity (W) after the elapse of time of 20000 hours, thatafter the elapse of time of 30000 hours, and that after the elapse oftime of 40000 hours, was indicated by a ratio when the refrigeratingcapacity (W) as an initial value was set to 100. Note that as the coldhead of each of the examples, a GM type cold head having therefrigerating capacity of 0.5 W under 4.2 K, was used.

TABLE 4 Refrigerating Capacity After After After Initial 20000 3000040000 Metal Mesh Material Value Hours Hours Hours Example 3 Sample C × 3100 94 90 85 Example 11 Sample B + Sample C 100 95 92 89 Example 12Sample B × 2 + Sample C 100 96 93 90 Example 13 Sample B + Sample C +100 96 93 90 Sample D

As can be understood from FIG. 4 , the cold head related to each of theexample 11 to the example 13 had a high maintenance ratio of therefrigerating capacity even after the elapse of time of 20000 hours,30000 hours, and 40000 hours. This indicates that even if theregenerator material is broken by the vibration, the clogging of themetal mesh materials is difficult to occur.

The maintenance ratio of the refrigerating capacity was high in the coldhead in which the metal mesh materials with different sizes of meshholes were stacked, as in the example 11 to the example 13. Thisindicates that in the metal mesh materials woven in thethree-dimensional manner, positions of the mesh holes of the overlappedmetal mesh materials are displaced so that the clogging of the metalmesh materials is suppressed.

Further, regarding cases where the example 3, and the example 11 to theexample 13 were employed for pulse type cold heads (having arefrigerating capacity of 0.5 W under 4.2 K), the maintenance ratio ofthe refrigerating capacity after the cold heads were continuouslyoperated was examined. Results thereof are presented in Table 5.

TABLE 5 Refrigerating Capacity After After After Initial 20000 3000040000 Metal Mesh Material Value Hours Hours Hours Example 3 Sample C × 3100 96 93 90 Example 11 Sample B + Sample C 100 98 96 92 Example 12Sample B × 2 + Sample C 100 98 97 95 Example 13 Sample B + Sample C +100 98 97 95 Sample D

From FIG. 5 , it was confirmed that excellent characteristics wereobtained also when the pulse type cold heads were used. This indicatesthat even if the regenerator material is broken by the vibration, theclogging of the metal mesh materials is difficult to occur. Note thatwhen the GM type and the pulse type were compared, the deteriorationrate of the refrigerating capacity in the pulse type was lower than thatof the GM type, since the vibration was smaller in the pulse type thanthat in the GM type.

Note that the above-described embodiments have been presented by way ofexample only, and are not intended to limit the scope of the inventions.Indeed, the novel embodiments described herein may be embodied in avariety of other forms; furthermore, various omissions, substitutionsand changes in the form of the embodiments described herein may be madewithout departing from the spirit of the inventions. The accompanyingclaims and their equivalents are intended to cover such forms ormodifications as would fall within the scope and spirit of theinventions.

What is claimed is:
 1. A method for manufacturing a cold head,comprising: preparing a regenerator container; filling a firstregenerator material particle group having an average particle size in arange from 200 μm to 380 μm in the regenerator container; filling aplurality of metal mesh materials on the first regenerator materialparticle group in the regenerator container; and filling a secondregenerator material particle group having an average particle size in arange from 200 μm to 380 μm on the plurality of metal mesh materials inthe regenerator container so that the second regenerator materialparticle group is partitioned with the first regenerator materialparticle group by the plurality of metal mesh materials, wherein each ofthe plurality of metal mesh materials has quadrangular mesh holes eachhaving a length of a long side in a range from 1/10 to ½ of each of theaverage particle sizes of the first and second regenerator materialparticle groups, wherein each of the plurality of metal mesh materialshas metal wires woven in a mesh form, and each of the metal wires has awire diameter in a range from 20 μm to 90 μm, wherein the plurality ofmetal mesh materials are stacked so that positions of the mesh holes inthe plurality of the metal mesh materials do not match, and wherein alength of one side of each of the mesh holes in one of the plurality ofmetal mesh materials is different from a length of one side of each ofthe mesh holes in another one of the plurality of the metal meshmaterials.
 2. The method according to claim 1, wherein one of theplurality of metal mesh materials comprises a copper mesh material. 3.The method according to claim 1, wherein the second regenerator materialparticle group has a different kind of material with the firstregenerator material particle group.
 4. The method according to claim 1,wherein each of the first and second regenerator material particlegroups is at least one selected from the group consisting of a leadregenerator material particle group, a holmium-copper regeneratormaterial particle group, an erbium-nickel regenerator material particlegroup, an erbium-cobalt regenerator material particle group, agadolinium oxysulfide regenerator material particle group, and agadolinium-aluminum oxide regenerator material particle group.
 5. Themethod according to claim 1, further comprising: filling a plurality ofmetal mesh materials on the second regenerator material particle groupin the regenerator container; and filling a third regenerator materialparticle group having an average particle size in a range from 200 μm to380 μm on the plurality of metal mesh materials in the regeneratorcontainer so that the third regenerator material particle group ispartitioned with the second regenerator material particle group by theplurality of metal mesh materials.
 6. The method according to claim 1,wherein a proportion of particle in which a form factor R represented byL²/4πA exceeds 1.5 is 5% or less in each of the first and secondregenerator material particle groups, where L is a circumferentiallength of a projected image of the regenerator material particle, and Ais an actual area of the projected image.
 7. The method according toclaim 1, wherein each of the average particle sizes of the first andsecond regenerator material particle groups is 200 μm or more and 300 μmor less; and wherein a number proportion of particle having a particlesize which falls within a range of 150 μm or more and 350 μm or less is95% or more in each of the first and second regenerator materialparticle groups.
 8. The method according to claim 1, wherein the coldhead is configured to achieve a temperature of 10 K or less.
 9. Themethod according to claim 1, wherein the cold head is configured toachieve a temperature of 5 K or less.
 10. The method according to claim1, wherein the cold head is a GM type cold head.
 11. The methodaccording to claim 1, wherein the cold head is a pulse type cold head.12. The method according to claim 1, wherein the cold head is a Stirlingtype cold head.
 13. The method according to claim 1, wherein the coldhead is used for a superconducting magnet, an examination apparatus, ora cryopump.
 14. The method according to claim 13, wherein theexamination apparatus is a magnetic resonance imaging apparatus or anuclear magnetic resonance apparatus.